The choice between air and oxygen as a medium for oxidation has been explored for many industrial processes. For processes where air can be used for oxidation without adversely affecting the reaction chemistry, air is preferred over a pure oxygen feed for reduced capital and operating cost. The higher cost of oxygen results from the expense of separating oxygen from nitrogen, conventionally accomplished using either membranes or distillation. One example where the use of pure oxygen is preferred over air is in syngas production, which is the first step in the production of liquid hydrocarbons from natural gas using Fischer Tropsch synthesis. In addition to affecting the reaction chemistry, the use of air in the syngas generation requires nitrogen to be separated from unconverted natural gas. The separation of nitrogen from natural gas is cost-intensive, and is conventionally carried out using membranes or pressure swing adsorption.
A conventional method of capturing trace bromine in inert gas streams is to use an alkaline scrubber, for example, an aqueous NaOH solution. It is not easy to recover the captured bromine from the conventional scrubbing method. Other conventional methods include the use of sodium sulfite, sodium bisulfite, or an aqueous solution of calcium hydroxide, etc., each of which suffers from the same drawback that the captured bromine is difficult to recover.
Technology developed by GRT, Inc., of Santa Barbara, Calif., allows higher hydrocarbons to be synthesized from methane or natural gas by mixing the hydrocarbon(s) and halogen in a reactor to form alkyl halides and hydrogen halide, HX. The alkyl halides and HX are directed into contact with a metal oxide or similar material to form higher hydrocarbons and a metal halide. The metal halide is oxidized to metal oxide and halogen, both of which are recycled. When air is used for oxidation, the halogen that is generated contains inert gases such as nitrogen and carbon dioxide which, if not removed, would pass through the halogenation section and the coupling section where higher hydrocarbons are produced. In other technology developed by GRT, Inc., hydrocarbons are formed by reacting alkyl halides with halogen in the presence of a catalyst, HX is formed as a byproduct. To regenerate halogen for use in a subsequent cycle of the overall process, HX is oxidized with air or oxygen in the presence of a catalyst. For both types of technologies, it is desirable to separate the regenerated halogen from N2, CO2, and other light gases (such as unreacted light hydrocarbons), and water before the halogen is used in the next process cycle. For a cyclic or continuous process, such gases, particularly nitrogen, would rapidly accumulate if not separated A clear need exists for an efficient, cost-effective process of separating inert gases and other compounds from halogens.